Can I assume we are okay with Rise of Skywalker spoilers here?
My primary goal would be to smooth out the transitions between the three films. It feels like Abrams had an idea for how the trilogy should go, and Johnson had an idea for how the trilogy should go, and they were... uh... not on the same page.
One example is the handling of the archvillains. In "The Force Awakens," Supreme Leader Snoke is a big mystery villain, seen only in hologram and only briefly. One would expect "The Last Jedi" to elaborate on where he came from and what his deal was; instead, he cackled and gloated for a bit, and then he got whacked by Kylo Ren, and that was that. "Rise of Skywalker" then had the job of a) explaining where Snoke came from, and b) introducing Palpatine as the Real Final Boss, and c) doing all this super fast at the start of the movie so it could get on with the plot. This was not a good situation to be in.
So, I would insert a few scenes into "Last Jedi" indicating that Snoke is a puppet, created by some shadowy entity behind the scenes to serve as a figurehead for the First Order and to seduce Ben Solo to the Dark Side. And I would close the movie on Palpatine's laughter. That would set up "Rise" much better.
Then there are the romantic subplots. I'm pretty okay with how the Rey/Ren one turned out: There was obvious chemistry there, but it was just as obvious that the movies could not possibly end with Rey and Ren riding off into the sunset together--Kylo Ren killing his father was a point of no return for the character. So, Ren turns away from the Dark Side, gets one kiss with Rey, and falls over dead. Rey then goes and adopts herself into the Skywalker family to retroactively continue the tradition of Skywalkers kissing each other, and all is right with the world.
Unfortunately, Finn's romances did not get resolved nearly as well. In "Force Awakens," he is clearly crushing on Rey. Then in "Last Jedi," there is the start of something with Rose. This gets unceremoniously dropped in "Rise" and he is back to crushing on Rey, which... goes nowhere. This one is definitely on "Rise of Skywalker" to fix. Finn just needs a few scenes where he is going back and forth between his crush on Rey and his feelings for Rose, finally settles on Rose (I'm not really seeing the Rey/Finn pairing, Finn/Rose works much better), and they get a hug and kiss at the end.
And the little kid using the Force at the end of "Last Jedi"--that kid needs to show up in "Rise" somewhere, somehow, doing something. Or he needs to not be there in "Jedi." Pick one, don't care which.
There are a bunch of these little hiccups between movies (Rey's parentage is another), all of which could be relatively easily smoothed out to greatly improve the trilogy as a whole.
I also have some issues with "Last Jedi" specifically, but those only impact "Last Jedi" and don't interfere with the larger arc of the trilogy, so they are less important to address. Let's just say we would spend a lot less time in the casino if I were the boss of Star Wars.